Manual Section... (2) - page: mprotect

NAME

SYNOPSIS

DESCRIPTION

mprotect()
changes protection for the calling process's memory page(s)
containing any part of the address range in the
interval [addr, addr+len-1].
addr
must be aligned to a page boundary.

If the calling process tries to access memory in a manner
that violates the protection, then the kernel generates a
SIGSEGV
signal for the process.

prot
is either
PROT_NONE
or a bitwise-or of the other values in the following list:

PROT_NONE

The memory cannot be accessed at all.

PROT_READ

The memory can be read.

PROT_WRITE

The memory can be modified.

PROT_EXEC

The memory can be executed.

RETURN VALUE

On success,
mprotect()
returns zero.
On error, -1 is returned, and
errno
is set appropriately.

ERRORS

EACCES

The memory cannot be given the specified access.
This can happen, for example, if you
mmap(2)
a file to which you have read-only access, then ask
mprotect()
to mark it
PROT_WRITE.

EINVAL

addr is not a valid pointer,
or not a multiple of the system page size.

ENOMEM

Internal kernel structures could not be allocated.

ENOMEM

Addresses in the range
[addr,
addr+len]
are invalid for the address space of the process,
or specify one or more pages that are not mapped.
(Before kernel 2.4.19, the error
EFAULT
was incorrectly produced for these cases.)

CONFORMING TO

SVr4, POSIX.1-2001.
POSIX says that the behavior of
mprotect()
is unspecified if it is applied to a region of memory that
was not obtained via
mmap(2).

NOTES

On Linux it is always permissible to call
mprotect()
on any address in a process's address space (except for the
kernel vsyscall area).
In particular it can be used
to change existing code mappings to be writable.

Whether
PROT_EXEC
has any effect different from
PROT_READ
is architecture- and kernel version-dependent.
On some hardware architectures (e.g., i386),
PROT_WRITE
implies
PROT_READ.

POSIX.1-2001 says that an implementation may permit access
other than that specified in
prot,
but at a minimum can only allow write access if
PROT_WRITE
has been set, and must not allow any access if
PROT_NONE
has been set.

EXAMPLE

The program below allocates four pages of memory, makes the third
of these pages read-only, and then executes a loop that walks upwards
through the allocated region modifying bytes.

An example of what we might see when running the program is the
following: